APS are solid state imagers where each pixel contains a photo-sensing means, reset means, a charge transfer means, a charge to voltage conversion means, and all or part of an amplifier. One undesirable phenomenon that can occur with solid state image sensors is referred to as blooming. This occurs when an extremely bright region in an image produces excess photoelectrons that can traverse the isolation regions that surround the photodetector in the pixel in which they were created and end up in other pixels or charge transfer regions within the image sensor and corrupt the photoelectron packet that was created by the irradiance incident upon the other pixels. Prior art APS devices have provided protection against blooming by keeping either the transfer gate and/or reset gate "off-level" electrostatically deeper than zero volts so that if more photoelectrons are generated than that which can be held by the photodetector, the excess photoelectrons can spill over the transfer gate and/or reset gate into the floating diffusion and/or reset supply. This approach has the disadvantage of diminishing the amount of photoelectrons that can be held by the photodetector, and in the case of a transfer gated pixel architecture, does not provide any blooming protection during read out of the sensor. Blooming protection can also be provided by inclusion of a lateral overflow drain (LOD) and lateral overflow gate (LOG) or a vertical overflow drain (VOD) within each pixel. Use of LOD and LOG has the disadvantage of decreasing fill factor. Inclusion of a VOD requires a more complex and thus a higher cost process.
In addition, APS devices have been operated in a manner where each line or row of the imager is reset, integrated and read out at a different time interval than each of the remaining lines or rows. In other words, the image capture for each row is done sequentially with the image capture for each row temporally displaced from every other row, with each row having the same integration time. Hence if one were reading out the entire imager, each line would have captured the scene at a different point in time. Since illumination conditions can and do vary temporally, and since objects in the scene may also be moving, this method of read out can produce line artifacts in the resulting representation of the image. This limits the usefulness of APS devices in applications where high quality motion or still images are required. In U.S. Pat. No. 5,986,297, entitled COLOR ACTIVE PIXEL SENSOR WITH ELECTRONIC SHUTTER, ANTIBLOOMING AND LOW CROSS-TALK, BY Robert M. Guidash, et al., disclosed is a means to mitigate these artifacts by performing frame integration, followed by row at a time readout. Since the readout time in this mode of operation can be as long as 30 msec. for the last row of pixels being read out, it is even more important to provide blooming protection during read out.
From the discussion above, it is evident that there is a need to provide blooming protection during integration and read out that does not adversely affect the fill factor of the pixel.